1. Field of the Invention
This invention relates in general to methods and apparatus for cleaning absorption columns, and more particularly, to methods and apparatus for cleaning nitric acid absorption columns.
2. Prior Art
In many industrial processes it is desirable to treat a dilute (20%) nitric acid stream to form a more concentrate (typically 59%, 70%, or 92%) nitric acid stream. This is accomplished through the use of an absorption column. Typically, such columns may be up to 200 feet tall and 10-15 feet in diameter. There will be a sparger assembly at the top of the column to introduce the dilute nitric acid, and a sparger assembly at the bottom of the column to introduce nitrous gases. Inside the column will be a series of bubble caps or sieve trays wherein each tray holds multiple rows of coils and within each row are multitudes of three-quarter inch or one and one-quarter inch diameter cooling coils. The number of coils per tray will vary depending on the cooling requirements of the column and may depend on such parameters as feed stock and column design. Typically, there are 4-36 coils per tray and 24-48 trays in a column. These cooling coils loop around the bubble caps located on the trays inside the tower and are designed to cool the liquids and gases passing through the tower. To accomplish this, water is pumped from a distant cooling tower into a vertical inlet or supply header standing beside the tower. Horizontal branches or nozzles are located on the header opposite the coil ends protruding from the sidewall of the tower, and the water is introduced into the coils via connecting hoses between the inlet header nozzles and the inlet ends of the coils. The water then flows through the coil and exits out of the tower, through return hoses into the vertical return header nozzles, and then finally back to the water cooling tower. Water is the usual cooling fluid circulated in the coils, although chilled brine may be used, particularly in the upper section of the column.
Because the efficiency of the absorption tower depends upon control of the heat of reaction taking place within the column it is important that one can control the amount of water circulating within the coils. However, several problems occur which can restrict the control of water flowing through the coils. It is not uncommon that wood splinters and other solid particles in the water cooling tower enter the water stream circulating within the coils. When this happens, it is possible for a coil or coils to become partially or completely plugged. In addition, the coils may develop leaks which allow nitric acid to enter the water stream and flow back to the water tower. When this acidic contamination to the cooling water system occurs, it is usual to add a base or alkaline material to the water to neutralize the effects of the acid on other parts of the water cooling system. The reaction of alkaline and acid in turn results in the formation of precipitates such as iron that will coagulate and, eventually, partially or completely plug various coils within the absorption tower. Finally, iron corrosion may develop and expand within the iron supply and return header nozzles which effectively restricts flow through the coils.
After a period of time it becomes necessary to repair leaking coils, as well as to clean out the plugged coils and header nozzles. The two most common methods of cleaning out the plugged coils include: (i) accessing the coils one at a time from outside of the column and trying to blow air or fluid through the coil under sufficient pressure to force out any material causing a blockage, and (ii) acid treating the coils by pumping acid through all of the coils at one time. Both methods have significant drawbacks. First is that both require a shutdown of the absorption tower for long periods of time while the repairs and cleaning are being carried out. Shutting down the absorption column effectively shuts down the nitric train because federal environmental air emissions regulations would prohibit the air emissions that would occur if the nitric train was not shut down. There may be 300-800 parallel coils within the column. In addition, each coil will contain multiple bends. Thus, the distance the entrapped air and reaction gases within the coils must flow to be displaced is great. This requires the use of acid in large volumes and under high pressures to flush the coils. Even then there is no assurance of opening the plugged coils a the acid will only seek the path of least resistance. Pumping through all the tubes at once results in the necessity of using large pumps, and creates a dangerous operation because of the possibility that connecting hoses, particularly if they are old hoses, may be blown off by the pressure. As a result of feeding so many coils in parallel, it is common that after an attempted acid cleaning, a substantial amount of acidic sludge may accidentally remain in the coils which then may be difficult to remove.
Testing for leaks on stream creates additional problems, not the least of which is that in many cases it is not readily apparent where a particular leaking coil is located, nor where its outlet exits the column. Thus, to plug-off a particular leaking coil requires that one first determine where the particular coil exits. One method used is extracting liquid samples through hypodermic needle syringes inserted into the return hoses. This can be a very time consuming task when the tower is on-stream, considering that there may be up to 36 coils per tray, and as many as 48 trays per column.